1. Field of The Invention
The invention relates to methods and procedures for the purification of bisphenol-A, and more particularly relates to purification by recrystallization from aqueous dispersions.
2. Brief Description of Related Art
Bisphenol-A [2,2-bis (4'-hydroxyphenyl)propane] is a valuable aromatic compound produced commercially by the reaction of a stoichiometric excess of phenol with acetone. In the course of manufacture, the desired bisphenol-A is precipitated as a crystalline adduct with unreacted phenol, including contaminant by-products and color bodies. Purification is desirable for many uses of the bisphenol-A, where the phenol and other contaminants would be objectionable.
Numerous purification procedures have been postulated and practiced to obtain bisphenol-A of high purity. Representative of such procedures are those described in the Japanese KOKAI PATENT APPLICATION NO. HEI 5[1993]-97746. According to this reference, the bisphenol-A/phenol adduct initially precipitated can be purified through a multi-stage crystallization procedure, interspersed with melting and separations. The reference teaches that such multi-stage recrystallizations do not completely remove substances that cause color, and the product crystals are still colored and not acceptable for all uses. The reference teaches washing the wet cake of powdered adduct with an organic solvent for phenol at a temperature of 60.degree.-120.degree. C., removing adsorbed solvent and drying the cake.
The multi-stage crystallizations as taught in U.S. Pat. No. 524,309 collectively referred to in the artisans language, as "melt crystallization" is a multi-stage purification process. The impurity "distribution coefficient" or "separation factor" achieved during each stage of separation is critical in determining the number of separation stages required to reach the desired product purity.
The present invention offers a process with an impurity "separation factor" greater than the prior art Melt Crystallization process, resulting in a process which achieves the desired purity in fewer stages.
Additionally, the temperatures at which this new purification process are carried out are significantly lower than those required in the prior art Melt Crystallization process. This fact, along with the potential for reducing energy consumption, makes it possible to substitute water and steam alternately, as the Heat Transfer Medium (HTM) in place of Therminol or Dowtherm type oils. These oils are very expensive, are environmentally troublesome, and are toxic to the waste water treatment Biomass.
The process of the invention produces a highly purified bisphenol-A in a high first pass yield, without the need for distillation, slurry handling, rotary vacuum filters, centrifuges, or the use of organic solvents.
The lower process temperatures employed in the process of the invention are due to the fact that bisphenol-A is first saturated with water. Bisphenol-A in relatively pure form occurs as crystals or flakes with a melting point (solidification range) of from about 150.degree. to 155.degree. C. The solid is practically insoluble in water.
However, in the presence of at least an equimolar proportion of water (7.3 percent by weight) the melting point of bisphenol-A is lowered to about 96.degree. C. At this temperature, the bisphenol-A forms an "oil" which is heavier than and also immiscible with water. A separate layer does not form in the mixture however, until the water saturation level is exceeded.
Advantageously, the amount of water included in the mixture with the "oil" or melted bisphenol-A is within the range of from about 20 percent by weight to 60 percent by weight of the total mixture, preferably 30 to 50 percent. The bisphenol-A/water "oil" can be maintained at a temperature of circa 98.degree. C. under atmospheric pressures (preferably at an elevated pressure of circa 1 to 10 PSIG).
Cooling of this mixture of essentially immiscible phases, with vigorous agitation, results in the formation of orthorhombic crystals of BPA which can be separated on a filter. The impurity laden fraction, which has been separated from the BPA is deposited as a "tacky" viscous "oil" on the surfaces of the BPA crystals. This is the basis of another purification procedure (described in the U.S. Pat. No. 3,326,986) where an organic solvent is used to wash the crystal cake, to solubilize the viscous coating of impurities. The process requires an organic solvent wash, the use of filters, centrifuges, slurry handling, and drying. Many of these steps and solvents are avoided by the improved process of the present invention.